This invention relates to electronic communication systems, and more particularly to a system and method for a handshake signaling protocol for digital subscriber line modems and similar systems.
A significant amount of effort has been undertaken to use existing telephone lines for high speed data communications. As part of this effort, a number of Digital Subscriber Line (DSL) systems have been proposed. For example, a version known as Asymmetric Digital Subscriber Line (ADSL) provides a system that applies signals over a single twisted-wire pair that supports xe2x80x9cplain old telephone servicexe2x80x9d (POTS) and high-speed duplex (simultaneous two-way) and simplex (from a network to the customer installation) digital services. Other DSL services are Very high speed DSL (VDSL) and Symmetric High speed DSL (SHDSL). A DSL system essentially encodes high rate digital data as analog signals using special modems. Proposed international standards for ADSL are G.dmt (based on the ANSI T1.413 Issue 2 ADSL Standard (1998)), and G.lite (based on G.dmt but with a lower maximum data rate and simpler implementation). Both of these standards incorporate another proposed international standard, G.hs (for G.handshake), as an essential part of their start-up procedures.
It has been proposed that a common handshake protocol, G.hs, be used for capabilities exchange between all DSL modems defined by the ITU. G.dmt and G.lite are the first examples of this commonality of use. The need for G.hs is to allow different DSL standards to be offered by modem vendors and service providers in such a way that varying applications and operating environments may dictate which DSL standard is chosen to be used on a case by case basis. As such, G.hs must be made simple enough that it may be implemented on a variety of platforms and flexible enough that it can continue to be useful as new DSL modems are specified.
Currently, G.hs is an evolving standard, and is not yet completely specified. However, it is proposed that G.hs will define how modem connections can be initiated, modem capabilities exchanged, and a final modem configuration negotiated. In terms of functionality, G.hs is to be similar to the well known V.8bis standard. The V.8bis standard is an established handshake protocol used for voice band services and employs an extensible tree structure which is not only efficient with respect to channel utilization but is easily expandable to support new services. These are seen as important qualities which should be incorporated into the evolving G.hs standard.
G.hs is a physical layer and messaging protocol specification used to establish a connection and negotiate a common mode of operation between two modems at either end of a subscriber line without any a priori knowledge of the capabilities of each end. However, different DSL systems operate in different frequency bands. Thus, since neither end knows what capabilities and frequency bands are supported at the other end, coordinating the G.hs transmit and receive spectrums such that they are not mutually exclusive and restrictive is difficult. This becomes an issue when a central office DSL modem must be able to adapt its transmission protocol to different subscriber modems. Indeed, any device capable of operation in different frequency bands would encounter difficulties in the initial contact with other devices if G.hs is not carefully designed. Such devices include both those that support multiple DSL standards and those that support variants, or xe2x80x9cflavorsxe2x80x9d, of a single DSL standard that have different frequency band allocations.
Accordingly, for the physical layer modem part, it is advantageous to define multiple tones in several specific frequency bands in order to account for the various DSL flavors which may be supported (i.e., G.lite in a Japanese ISDN environment requires a different frequency spectrum than G.lite or G.dmt in a European environment). One approach would be to implement G.hs transceivers such that they are capable of transmitting and receiving over many different frequency bands. However, the addition of each new frequency band increases the cost of implementation and as new frequency bands are added in the future, older G.hs equipment would become obsolete.
One proposal suggests probing each of the bands at the transmitter and receiver, either in serial or parallel, to determine a mutual spectrum. However, this approach is somewhat time consuming, relatively complex, and does not allow existing G.hs equipment to support additional frequency bands which may be introduced in the future. This approach also burdens future products with a possibly onerous backwards compatibility constraint.
The inventors have realized that a simple, expandable approach to the use and assignment of tones within frequency bands for G.hs intercommunication would be desirable. The present invention provides an architecture and particular embodiments of such a solution.
The invention provides a system and method for a handshake protocol for digital subscriber line modems and similar telecommunication systems that makes beneficial use of aliasing to select signaling tones. In particular, the invention carefully selects set(s) of signaling tones (xe2x80x9ctone setsxe2x80x9d) at frequencies such that higher frequency tones would alias down and coincide with a base tone within the set after sub-sampling (possibly after a frequency shift), where sub-sampling is defined to be sampling at a rate lower than the Nyquist sampling rate of the tones of interest. Thus, rather than filtering out higher frequencies to avoid aliasing, aliasing is exploited to use these higher frequencies. This technique would allow a receiver to detect and decode the higher frequency tones at lower frequency tone locations or bands. A related innovation is a band set, which includes one or more tones within a particular frequency band, where typically each tone in a band set corresponds to a different tone set.
Exploiting the effects of aliasing eliminates the need to search for a compatible spectrum, simplifies receiver design, provides flexibility, scalability and future-proofing, and even allows handshaking sessions to be established across mutually exclusive spectrums. The invention is particularly useful for DSL systems, and in particular is well suited for the proposed G.hs standard.
In one aspect, the invention includes a system, method, and protocol for communicating capabilities between a first telecommunication unit and a second telecommunication unit, each having a receiver, including a first tone set including at least one tone for signaling from the first telecommunication unit to the second telecommunication unit, the first tone set including a base tone such that all other tones within such first tone set alias onto the base tone when appropriately sub-sampled by the receiver within the second telecommunication unit, possibly after a frequency shift.